The rule you reference is the current, legally enforceable version of ADAAG. The new ADA-ABA Guidelines, which will replace ADAAG once adopted by the appropriate federal agencies such as DOJ, says "705.1.3 Contrast. Detectable warning surfaces shall contrast visually with adjacent walking surfaces either light-on-dark, or dark-on-light." Those rules are at www.access-board.gov/ada-aba/final.htm. The Access Board is also working on rules specific to the public right of way since the current rules are geared more to building construction. They published a revised draft ROW rule on Nov. 23, 2005. You can find it at www.access-board.gov/prowac/index.htm. There's still not a lot of measurable guidance on contrast.... the draft ROW rule says "R304.1.3 Contrast. Detectable warning surfaces shall contrast visually with adjacent gutter, street or highway, or walkway surfaces, either light-on-dark or dark-on-light."